6P Films Research Articles

Texture and morphology of ZnO grown on nanocrystalline p-sexiphenyl thin films

Growth of ZnO by molecular beam epitaxy on submonolayer p-sexiphenyl (6P) films consisting of differently shaped and terminated nanocrystals is studied. The morphology and crystallographic structure of ZnO are analysed by cross-sectional transmission electron microscopy (TEM). The inorganic-on-organic interface is remarkably abrupt and the 6P nanocrystals remain unimpaired as revealed by TEM and complementary photoluminescence measurements. The wurtzite ZnO overlayer shows a distinct columnar structure with a preferential c-axis alignment. The influence of the ZnO growth direction and 6P crystal termination on the structural properties is discussed.

Crystal growth of para-sexiphenyl on clean and oxygen reconstructed Cu(110) surfaces

The formation of crystalline para-sexiphenyl (6P) films on Cu(110) and Cu(110)-(2 × 1)O (Cu-O) has been studied by low energy electron diffraction, X-ray absorption spectroscopy and both in situ and ex situ X-ray diffraction methods to elucidate the transition from the initial monolayers to crystalline thin films. It is found that, for Cu-O, a single and, for Cu(110), a double wetting layer is formed which then acts as a template for the subsequent 3D crystal growth. For both substrates the orientation of the long molecular axes of the 6P molecules in the first layers is conserved for the molecules in the bulk crystals growing on them. The main difference between both systems is that on Cu-O the first monolayer assembles in a form close to that of a 6P bulk plane which can be easily continued by crystallites grown upon them, while on the Cu(110) surface the 6P mono- and bi-layers differ substantially from the bulk structure. The bi-layer forms a complex periodically striped phase. Thin 6P films grow with the 6P(203) crystal plane parallel to the Cu-O substrate surface. For this orientation, the 6P molecules are stacked in layers and the molecules demonstrate only one tilt of the mean molecular plane with respect to the sample surface. On clean Cu(110), a more complex 6P(629) plane is parallel to the substrate surface and this orientation is likely a consequence of the super-molecular long-range periodicity of the second molecular layer striped phase.

Characterization of Organic/Metal Interfaces Using Angle-Resolved X-ray Photoelectron Spectroscopy

Interfaces between organic films and gold substrates were investigated by angle-resolved X-ray photoelectron spectroscopy (ARXPS). We examined the emission angle dependence of the peak position of the ARXP spectra in the Au 4f region, and firstly demonstrated that the ARXPS method enable the observation of the electronic structure of the surface layer of the Au film. By comparing two types of organic adsorbed layer on the Au substrate between chemisorption (self-assembled monolayer) and physisorption [p-sexiphenyl (6P)], we found that the electronic structure of the first Au layer was changed by the chemical interaction between the Au top layer with the organic adsorbates. We discussed ARXPS results of Au deposition on the 6P film to confirm Au diffusion in the organic film. These results indicate that ARXPS can elucidate the electronic structural changes of the surface layer of a substrate and the penetration of metals in organic/metal systems.

Search for a wetting layer in thin film growth of para-hexaphenyl on KCl(001)

The non-existence of a strongly bound wetting layer for the system para-hexaphenyl (6P)-KCl(001) was verified by thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). The 6P films were grown by physical vapour deposition under ultrahigh vacuum conditions at 400 K. TDS showed just a single desorption peak even down to a coverage of 0.1 nm mean film thickness. The heat of evaporation for 6P was determined to 2.3 eV. From the change of the XPS K2p substrate signal as a function of the mean film thickness one can conclude that a strong de-wetting exists at the very beginning of the 6P layer growth at 400 K. Additional investigations with atomic force microscopy reveal that at this initial stage the film grows in a form of needle like islands with a high aspect ratio and subsequently terraced mounds of 6P are formed. No dissociation of 6P on KCl was observed.

Electrical transport properties of hot wall epitaxially grown para ‐sexiphenyl nano‐needles

AbstractHighly ordered anisotropic para ‐sexiphenyl (6P) films with nano‐needle structures, grown by hot wall epitaxy (HWE) on monocrystalline mica substrates, were prepared. The current–voltage (I–V) behaviour of gap‐type devices with current parallel and perpendicular to the needle axis was studied. From a space‐charge‐limited current (SCLC) regime in the I–V curves the charge carrier mobility was determined. When measured along the direction of needles, an effective charge carrier mobility as high as 14 cm2/Vs was obtained. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hexaphenyl thin films on clean and carbon covered Au(1 1 1) studied with TDS and LEED

The adsorption and growth of ordered para-hexaphenyl (6P) films have been investigated both on clean and partially carbon pre-covered Au(1 1 1) single crystal surfaces by thermal desorption spectroscopy (TDS) and low energy electron diffraction (LEED) under ultra-high vacuum conditions. The existence of a distinct first and second monomolecular 6P layer that clearly separate from the multilayer regime, which comprise lying molecules with respect to the substrate surface, could be inferred from TDS. For both the 6P mono- and multilayer grown on pure Au(1 1 1) the desorption energies have been determined based on experimental TDS data. In particular, for the monolayer regime a coverage dependence of the desorption energy has been found, which is attributed to repulsive interactions between neighbouring 6P molecules adsorbed on the gold surface. The existence of well-ordered film structures could be inferred from LEED for half monolayer and full monolayer thick 6P films. Based on the LEED and TDS data, structural models are presented for these highly ordered organic films. Multi-step dehydrogenation of 6P molecules adsorbed on clean Au(1 1 1) surfaces is reported for temperatures above 650 K together with experimental evidence for the existence of a regular overlayer composed of partially dehydrogenated polycyclic aromatic hydrocarbon (PAH) intermediates. A quite different adsorption/desorption kinetics and film growth has been observed for 6P films grown on carbon pre-covered Au(1 1 1) surfaces.

Molecular Rearrangement and Optical Property Changes of p-Sexiphenyl Vacuum-Deposited Film Induced by Rubbing

Structural and optical property changes of p-sexiphenyl (6P) films induced by rubbing were investigated. 6P molecules vapor-deposited on a fused silica glass adsorbed obliquely but had no in-plane orientation. After rubbing, the film surface became smooth and the electron microscopic analysis revealed that the longitudinal direction of the 6P molecules is parallel to the substrate surface and oriented uniaxially along the rubbing direction. The absorption spectrum changed corresponding to the orientation change of the 6P molecules. The 6P molecules grew homoepitaxially on the rubbed film, and the film exhibited a high anisotropic optical property (dichroic ratio: 9.6) when deposition was performed at a low substrate temperature (RT) and a high deposition rate (5 nm/min).

Efficient polarized blue electroluminescent device using oriented p-sexiphenyl thin film

Abstract Highly oriented p -sexiphenyl (6P) film was successfully prepared by epitaxial-growth on rubbed 6P film on hole-transporting poly(9,9 ′ -dioctylfluorene) (POF). Using the epitaxial-growth film of 6P, we prepared tri-layered device, ITO/POF/epitaxial-growth film of 6P/electron-transporting ozadiazole derivative (OXD7)/LiF/A1. The device with the epitaxial-growth film of 6P as emissive layer exhibited highly polarized and efficient blue electroluminescence (EL). EL peaking around 430 nm corresponded well to photoluminescence of 6P film. The maximum value of polarization ratio I p / I s of the blue EL was 37, where I p and I s are EL intensity parallel and perpendicular to the orientation direction, respectively. The values of maximum luminance L max and maximum current efficiency η max of the tri-layered device were enhanced comparing to those of a double-layered device ITO/6P/OXD7/LiF/Al; L max =146 cd m −2 and η max =0.41 cd A −1 for the tri-layered device whereas L max =32 cd m −2 and η max =0.07 cd A −1 for the double-layered device.

Metal-on- p-sexiphenyl films studied by electron spectroscopies

Systems formed by depositing metals (Au and Mg) on p-sexiphenyl (6P) films were studied by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and metastable atom electron spectroscopy (MAES). The results of MAES indicate the penetration of metal into 6P for both Au and Mg deposition. Especially, 6P surface remains uncovered even when the average thickness of deposited Au reaches 125nm. An anomalous shift of spectra to the lower kinetic energy side, possibly due to some type of sample charging effect, was induced by the deposition of small amount of Au on 6P film, although conducting material was deposited.

Electronic Properties of the Interfaces Between the Wide Bandgap Organic Semiconductor Para-Sexiphenyl and Samarium

Ultraviolet photoelectron spectroscopy was used to determine the energy level alignment at the interfaces formed by either the deposition of para-sexiphenyl (6P) onto samarium thin films, or by growing samarium on 6P films. We find that for both cases (6P on Sm, and Sm on 6P) the interaction at the interface is weak (physisorption), and the interfacial dipole is smaller than 0.2 eV. This weak interaction enabled us to demonstrate that small morphological changes in the substrate Sm film lead to a reversal in the direction of the interfacial dipole. For native silicon oxide substrates the appearance of a photoemission signal more than 1 eV above the Fermi level is observed, and explained by the energy level alignment process for wide bandgap organic semiconductors. Additionally, we demonstrate that large lateral vacuum level shifts (ca. 1 eV) can be locally realized on an inhomogeneous sample.

Electronic Properties of the Interfaces Between the Wide Bandgap Organic SemiconductorPara-Sexiphenyl and Samarium

Ultraviolet photoelectron spectroscopy was used to determine the energy level alignment at the interfaces formed by either the deposition of para-sexiphenyl (6P) onto samarium thin films, or by growing samarium on 6P films. We find that for both cases (6P on Sm, and Sm on 6P) the interaction at the interface is weak (physisorption), and the interfacial dipole is smaller than 0.2 eV. This weak interaction enabled us to demonstrate that small morphological changes in the substrate Sm film lead to a reversal in the direction of the interfacial dipole. For native silicon oxide substrates the appearance of a photoemission signal more than 1 eV above the Fermi level is observed, and explained by the energy level alignment process for wide bandgap organic semiconductors. Additionally, we demonstrate that large lateral vacuum level shifts (ca. 1 eV) can be locally realized on an inhomogeneous sample.

Bipolaron formation in para-sexiphenyl thin films upon Cs doping

The semiconducting electroluminescent oligomer para-sexiphenyl (6P) is a promising candidate for application as the active layer in organic light emitting devices. The interaction of Cs atoms evaporated (in a step-wise manner) in ultrahigh-vacuum on thin 6P films has been studied with synchrotron ultraviolet photoelectron spectroscopy. No formation of metallic Cs or of new chemical compounds has been evidenced, but it appears that a doping of the oligomer occurs for the whole range of Cs concentrations we studied. Our data show a progressive growth of new density of states in the (former empty) band gap. Upon progressively increased doping, the material work function decreases, its Fermi-energy remains well below the newly occupied electronic states measured at 5.8 and 3.6 eV (energies relative to the vacuum level). These observations are interpreted as bipolaron formation in the charged 6P material.

Bipolaron: The Stable Charged Species in n-Doped p-Sexiphenyl

The change in the valence electronic structure of thin films of the electroluminescent material p-sexiphenyl (6P) has been investigated as a function of the deposition of an alkaline earth metal under ultrahigh vacuum conditions. This was done by in situ evaporation of increasing amounts of calcium onto the 6P film and recording the photoemission spectra. Two different substrates for the 6P films were used, namely Au and Ca. New emissions within the formerly empty energy gap of the pristine organic material were observed upon the metal deposition for both substrates. For 6P/Au this was accompanied by a significant shift of the Fermi level (EF) toward lower binding energy for increasing amounts of Ca. For low metal coverage the density of occupied valence states (DOVS) of the intragap emission was observed at EF, whereas for higher amounts of Ca coverage no DOVS are observed at EF. Commonly, this would be interpreted that first polarons (radical anions) are formed, and later on bipolarons (dianions) for hi...

P-Sexiphenyl/metal interfaces studied by photoemission and metastable atom electron spectroscopy

Abstract p -Sexiphenyl(6P)/metal (Au, Mg) interfaces were studied by ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and metastable atom electron spectroscopy (MAES). The vacuum level shifts ( Δ ) at the interface were observed in both 6P/Au ( Δ =−0.8±0.1 eV) and 6P/Mg ( Δ =−0.4±0.1 eV) systems. The MAES spectra indicate that the 6P molecules are on the substrate with their mean molecular plane parallel to the substrate surface in both systems, and that the 6P film on Mg substrate has some disorder. Thickness dependence of UPS, MAES, and XPS spectra suggests no significant chemical interaction for 6P on Au, Mg as well as Au, Mg on 6P interfaces. In the system of Au on 6P/Au, the spectral feature of 6P in MAES spectra did not disappear even when the thickness of evaporated gold reached 20 nm, indicating a penetration of Au atoms (or the clusters) into the 6P film.

Electronic structure of p-sexiphenyl — metal interfaces studied by electron spectroscopies

Abstract The electronic structure of p -sexiphenyl (6P) film on metal substrates (Au and Mg) was investigated by ultraviolet photoelectron spectroscopy (UPS), metastable atom electron spectroscopy (MAES), and X-ray photoelectron spectroscopy (XPS). From MAES spectra, we found that 6P molecules form an uniform monoiayer on Au substrate, while they may form an islands on Mg substrate. UPS spectra showed that the work function of the metal substrate was changed by deposition of 6P, indicating the formation of interfacial dipole layer. No chemical reaction between the metal and the 6P molecules was observed in the XPS spectra. We also investigated the reverse system, i.e. metal on the 6P film. In contrast to the formation of the abrupt interface for the 6P/metal system, the penetration of metal atoms occurs at the metal/6P interface.

Photoemission Study of Metal-Deposited p-Sexiphenyl Film

ABSTRACTMetal deposition on a p-sexiphenyl (6P) film was studied by ultraviolet photoelectron spectroscopy (UPS), metastable atom electron spectroscopy (MAES), and X-ray photoelectron spectroscopy (XPS). The deposited metals were Au, Mg, and several alkali metals (K, Na, Rb, and Cs). No chemical reaction between 6P and Au or Mg was observed in the measured spectra, while additional gap states appeared in the UPS and MAES spectra by deposition of the alkali metals. The diffusion of Au and Mg atoms into the 6P film was observed in the MAES spectra. We found the trend of the vacuum level shift is different between the systems of the 6P on the Au and its reversed systems (Au on the 6P film), suggesting the different formation of the interface depending on the deposition sequence.

The Valence Electronic Structure of N-Doped P-Sexiphenyl

ABSTRACTThin films of p-sexiphenyl (6P) were doped with increasing amounts of potassium in situ, and the change in the valence electronic structure of 6P upon the alkali metal deposition was followed with ultraviolet photoelectron spectroscopy. We observe the evolution of new intragap emissions, which are attributed to the formation of bipolarons, even for very low doping concentrations. The low binding energy intra-gap emission exhibits a pronounced asymmetric lineshape, in contrast to the findings when cesium is used as dopant. In order to investigate whether this lineshape is due to different emissive electronic species in the bulk and on the surface of the 6P film the take-off angle for the photoelectrons was varied. As no change in the lineshape is found when going from normal to near-grazing emission we can exclude that charged 6P molecules in the bulk and on the surface yield different valence electronic spectra. Therefore, the characteristic lineshape of the low binding energy emission is proposed to be related to the interaction of the doped organic molecule with the different counterions.

Epitaxial Thin Film Structures of Electroluminescent Materials

ABSTRACTDespite the fast-paced new developments and improvements of applications incorporating conjugated organic materials, rather little effort is made to obtain well-defined structures and thin films of these in order to exploit the highly anisotropic properties (e.g. conductivity, polarized light emission) of most conjugated organic materials. It has been shown that oligomers, which can easily be evaporated under ultrahigh vacuum conditions, exhibit heteroepitaxial growth on various substrates. Depending on the nature of the interaction of the organic molecules with the substrate different orientations can be obtained, as well as by simply changing the conditions of evaporation. Therefore, by going from amorphous or polycrystalline structures to highly textured or single crystalline ones new insights into the physical properties of the materials can be expected, and also new applications. We studied the formation of well-defined films of the electroluminescent oligomer p -sexiphenyl on various metal and semiconductor surfaces and determined the orientation of the molecules by means of low energy electron diffraction (LEED), infrared reflection absorption spectroscopy (IRRAS) and atomic force microscopy (AFM). Depending on the substrate material we found that the molecules are oriented either parallel or perpendicular to the substrate surface. In addition to that, highly oriented 6P films were doped with potassium, and the change in the molecular vibrational signature was investigated with IR-RAS.

Organic electroluminescent device with epitaxial p-sexiphenyl films

Organic electroluminescent (EL) behaviors were investigated with epitaxially oriented and disordered polycrystalline films of emissive p-sexiphenyl (6P) molecules. In comparison to the disordered polycrystalline 6P film, the epitaxial 6P film in which the molecular axis possessed lying orientation remarkably exhibited a high EL intensity at lower driving voltages, whereas the epitaxial 6P film with standing orientation emitted a weak intensity even at higher voltages.

Polarized electroluminescence from oriented p-sexiphenyl vacuum-deposited film

Oriented sexiphenyl (6P) film was prepared through epitaxial growth on rubbed substrates by vacuum deposition. Polarized absorption spectra demonstrated that the long axis of 6P molecule was highly oriented along the rubbing direction. Using the epitaxial growth film of 6P, we fabricated electroluminescent (EL) devices consisting of indium-tin-oxide anode, emissive layer of 6P, electron-transporting layer of an oxadiazole derivative, and MgAg alloy cathode. Owing to the orientation of 6P molecules, EL polarized in the rubbing direction was observed in the devices.